ACHEM EXAM 1

Accuracy

how close to true value

precision

how close values are to eachother

Methods for Accuracy

Reference Material (confidence interval)

Gold Standard (F test and T test)

Spiking (known analyte amount)

Sensitivity

how small changes in concentration result in big changes of signal

selectivity/specificity

ability of method to distinguish between analyte and other things

Robustness

ability of method to be unaffected by small changes in parameters

Method Validation

process of proving an analytical method is acceptable for its intended purpose

Method Validation Requirements

Accuracy, precision, specificity, robustness,range, limit of detection, limit of quantification, linearity

Instrument Precision

measure the same samples many times

Intra-Assay Precision

analyze same sample many times bit same person, instrument, and reagents in aliquots

Intermediate Precision

Analyze same sample many times but w/ different person, instrument, reagents, ady (within same lab)

Interlaboratory Precision

Analyzing same sample in different lab using same protocol

Horwitz Trumpet

As concentration decreases, the precision becomes worse

Determining specificity with Interference

10 to 100 x higher than analyte solution

Robustness Determination

change environmental conditions (1 mmol to 1.1 mmol reagent)

Limit of Detection

lowest limit to determine if analyte is present or not (3x standard deviation of blank)

Limit of quantificaiton

lowest limit that the concentration of analyte can be determined (10x stdev of blank)

Determining Range

concentration interval with acceptable quality characteristics

Reporting Limit

concentration below which regulations say that analyte reported as “ND” (not dtected) eben when it is observed

Results of Assay with ONLY Random errors

Gaussian Distribution

Coefficient of Variation (RSD)

100 x (stdev/mean)

Grubbs Test

How to determine outliers —> (questionable value- mean) / stdev —> Gcalc>Gtable BAD

HOW TO CALIBRATE SYSTEM

Calibration curve, standard edition method, internal standard

Calibration Curve

signal comes from just analyte (NO MATRIX EFFECTS)

Standard edition method

Matrix effects, marking or pretreatment

Internal Standard Method

typically Chromatography , when samples test during preparation if samples and changes of experimental conditions

Least Square Method

minimizes total residual error, line of best fit

Steps for Calculating Uncertainty

take 3+ trials, check for outliers, take average + standard dev, subtract blank, plot data, linst method w/ uncertainties , R² (1 non 9 digit)

STDev Rules

2 significant digits, round data point to same # decimals as stdev

Blank Solutions

prepared with same reagents + solvents to prepare standards and unknowns ,same matrix, but no analyte

Signal point standard addition method

I(x)/I(x+s)= [x]i /([x]f -[s]f)

Multiple Standard Edition Method

multiple flasks w/ same amount of unknown, add varying amounts of standard leaving 1 with only unknown, dilute samples to same final volume with other reagents

Internal Standard method

known amount of a compound that is added to a compound (not analyte), Ax/[x] =F (Ais/[IS])

response factor F —> relative response to analyte

Quality Assurance

what we do to get the right answer for our purpose —> check if product meets specific specifications

Use objectives

from which specifications for data quality can be derived

specifications

could include requirements for sampling, accuracy, precision, specificity, detection limit, standards, and blank values

Asssessment

process of 1) collecting data to show procedures are operating within specified limits an 2) verifying final results meet use objectives

Specifications include

sampling requirements, accuracy+precision, rate of false results, selectivity and sensitivity, acceptable blanks, spike recovery, calibration checks, quality control samples

Sampling requirements

representative samples and analyte must be preserved

Accuracy and Precision

within practical restraints what level if accuracy and precision meet requirements

False Positive

implies concentration exceeds limit when it is below

False Negative

implies concentration is less than limit when it is above

Blanks account for ___

intereference, traces of analyte in reagents, detect analyte from previous samples due to adhering to vessels or instruments

Method Blank

similar matrix sample run through whole method of analysis

Reagent Blank

just reagents that are used in samples

Field Blanks

follow all steps (including storage) and methods and reagents

Spike Recovery

ability of methods to differentiate known concentrations of samples, % recovery = (Cspike-Cunspiked)/Cadded x 100

How often should calibration checks be done?

1x per day (more often for less robust methods), initial calibration standards cannot be reused throughout day, calibration check w/ standards every ~10 samples

Quality Control

helps eliminate bias from analyte (solution of known concentration but not to the analyst)

Control Charts

visual representation of confidence intervals for measurements having a gaussian distribution (ACTION LINE mean ± 3stdev : Warning line mean ± 2 stdev)

Standard Operating Procedure

MUST BE FOLLOWED WITHOUT DEVIATIONS

What methods measure current?

Voltammetry and Amperometry

What methods measure voltage?

potentiometry

What methods measure resistance?

Spectrometry

Working Electrode

where redox reaction occurs, or interaction of analyte

Reference Electrode

Electrode w/ fixed reference potentital

Counter Electrode

electrode where opposite electrochemical process to close the circuit (CARBON + Platinum), BIGGER than WE

Metal Electrodes

electrode potential in redox reactions

Ion-selective Electrode

interaction of analyte (POTENTIOMETRY)

Indicator Electrodes

Gold, platinum, carbon —> needs to be inert (current cannot be contributed by electrides having a reaction)

Types of RE

Standard Hydrogen Electrode (has 0 potential, difficult to set up/use)

Silver/Silver chloride

Saturated calomel

Potentials

E = Ewe- Ere

Ion-Selective Electrodes

NO REDOX RXN —> interactions w/ analyte (liquid based I-S, solid-state I-s, compound electrode)

Potentiometry

Potential difference —> 2 reference electrodes

Ligand Selection

Forms complex based on charge and size

Glass Electrode

2 reference electrodes measure elec. potential, hydrated glass form gel, difference in concentration b/w inner/ outer solutions causes potential difference

Errors in pH Measurement

standards (±0.01), junction potential, junction potential drift, sodium error (>14), acid error (<1), equilibration time, hydration of glass, temperature, cleaning

Response of Electrode

E= constant -0.05916log[A]

Solid-State ISE

inorganic crystals (fluoride electrode), crystal of LaF3 doped w/ Eu2+ —> fluoride ions move to vacant spots and ump out of crystal

Liquid-Based ISE

hydrophobe membrane w/ ionophone (Ligand) is selective for analyte, responds to free/uncomplexed ions

Compound Electrodes

pH electrode surrounded by membrane, only analyte passes through, can be used to measure gas in solution (?), analyte changes pH of inner solution, detected by pH electrode

Solid-State Chemical Sensors

n-type (excess conduction e-, silicon- and phosphorus+) and p-type (excess holes , silicon + and aluminum-)

Field Effect Transistor (FET)

constructed of p-silicon base and 2 n-type regions

Electrolysis

the process in which a chemical reaction is forced to occur @ an electrode by an applied voltage

ELECTROLYSIS EQ

Ecell=E(cath)-E(anode)-IR-overpotential

Overpotential

voltage required to overcome the activation energy of an electrode rxn

ohmic potential (IR)

voltage needed to overcome internal resistance of the cell

Concentration Polarization

occurs when the concentration of electroactive species near an electrode is not the same as its concentration in bulk solution

Coulometry (chronoamperometry)

coulometric (constant current) titration the time needs to be complete reaction measures the number of electrons consumed

How can you reduce ohmic potential?

add salts

Amperometry

current @ WE is proportional to analyte concentration, potential is set to specific value and measured after addition of sample

Diffusion Current

limiting current determined by rate of diffusion

Mediator

employed to shuttle e- between electrode and analyte

Voltammetry

measures current, collection of method in which the dependence of current on the applied potential of WE is observed

Faraday Current

comes from redox rxn (WANT)

Capacative current

flow of ions to electrode whenever potential is changed

residual current

oxidation/reduction of other species in solution

stripping

concentrated analyte into film.single drop by reduction @ fixed voltage for fixed time

Spectrophotometry Eq’s

Transmittance =P/p0

Absorbance = -logT

What does choosing cuvette type depend on?

wavelength cuvette absorbs

S1

singlet excited state

T1

triplet excited state

S0

ground state

S1-S0

internal conversion, heat release

S1-T1

Intersystem conversion, phosphorescence

S1 —> ground

fluorescence

Difficulty frmo T1→S0

must switch spin state

Phosphorescence vs Fluorescence

Fluorescence has larger E gaps —> smaller wavelength

Phosphorescence slower

Luminescence

emission of light (fluorescence, phosphorescence, chemiluminescence, electro-chemi luminescence)

more sensitive than absorbance

What causes more fluorescnce?

rigid molecules